The present invention relates to optical data recording media with the optical state thereof variable by laser beam irradiation and, more particularly, to phase change optical disks.
Optical data recording media such as magneto-optical disks and phase change optical disks are well known in the art, which are capable of recording, erasing and reproducing the data by laser beam irradiation. Phase change optical disks permit ready construction of optical heads and ready over-writing. Researches and investigations for realizing higher density have been made.
For realizing higher density, the land/groove recording has been extensively researched, in which both land and groove are used for recording and reproduction. In the land/groove recording, reduction of cross-talk from adjacent land region or adjacent groove region is a serious problem. Regarding the cross-talk reduction, a technique of making the groove depth to be greater than a conventional value of .lambda./8 or a technique of removing influence of cross-talk in a signal processing, are well known in the art as described in Japanese Laid-Open Patent Publication No. 5-282705.
For obtaining equal recording and reproducing characteristics with both the land and groove regions based on the above techniques, it is necessary to make the optical phase difference .DELTA..phi.=.phi.a-.phi.c between crystal state and amorphous state of a recording layer of the phase change optical disk to be approximately 0 degree.
For further density increase, a technique of combining the land/groove recording and mark edge recording, in which the data are provided at opposite ends of the recording mark, is promising. Particularly, in applications to the image recording or the like, high data transfer rates are desired, which dictates realization of high linear speed mark edge recording. In the mark edge recording, it is important how to reduce jitter. It is well known in the art that in order to realize high linear speed mark edge recording with the phase change optical disk by reducing the jitter in the over-writing, it is necessary to make the light absorption coefficient Aa of crystal to be higher than the light absorption coefficient As of amorphous.
FIG. 12 is a schematic sectional view showing a prior art phase change optical disk. As shown, the phase change optical disk 30 is obtained by laminating on a substrate 12 a first dielectric layer 14, a recording layer 16, a second dielectric layer 18 and a reflective layer 32 in the mentioned order. The substrate 12 is of polycarbonate (PC), the first and second dielectric layers 14 and 18 are of ZnS-SiO2, the recording layer 16 is of GeSbTe, and the reflective layer 32 is of aluminum.
With the prior art phase change optical disk 30 as stated, however, the reflective layer 32 which is made of aluminum or like metal, hardly transmits light. Therefore, it is difficult to control the light absorption coefficient of the recording layer 16. In addition, under a condition that Ac&gt;Aa, it is difficult to increase (Rc-Ra) in order to ensure satisfactory signal quality or C/N or simultaneously meet .DELTA..phi.=0 in order to obtain the equal recording and reproducing characteristics with both land and groove.
FIG. 13 is a graph showing optical characteristics versus thickness of the first dielectric layer in the phase change optical disk shown in FIG. 12. Reference is now had to FIGS. 12 and 13.
Where the thickness of the first dielectric layer is in the neighborhood of 240 nm, the condition of Ac&gt;Aa is satisfied. However, .DELTA..phi. is greater than 70 degrees and cannot satisfy .DELTA..phi.=0. It is thus impossible to reduce cross-talk simultaneously in both the land and groove regions. Where the thickness of the first dielectric layer 14 is in the neighborhood of 170 nm, .DELTA..phi. is relatively small, i.e., .DELTA..phi..apprxeq.30 degrees. However, it is impossible to satisfy Ac&gt;Aa.
As shown above, it is difficult to apply the prior art phase change optical disk 30 for both the land/groove recording and the high linear speed mark edge recording, and it is impossible to realize further density increase.